Abstract: Magnitude and Timing of Displacement on San Andreas Fault in Southern California and Its Palinspastic Implications

P. L. Ehlig

The San Andreas fault and its branches have a total right slip of 310 km in southern California. This is shown by the offset of distinctive basement and volcanic terranes and formations containing clasts derived from them. The San Gabriel fault which was a segment of the original San Andreas fault began moving in the late Miocene (10-12 m.y. ago), and accumulated 60 km of right slip before becoming inactive in early Pleistocene time. The present segment of the San Andreas fault between Frazier Park and San Bernardino developed since the early Pliocene (6-8 m.y. ago) and has 250 km of right slip. A middle Miocene palinspastic reconstruction places the Soledad basin and San Gabriel Mountains adjacent to the Orocopia and northern Chocolate Mountains near the present Salton S a. Terrane west of the San Gabriel fault near Frazier Park originated south of the Soledad basin. The youngest formation with 310 km of offset is the Miocene Caliente Formation west of Frazier Park, the offset equivalent of the Mint Canyon Formation of Soledad basin. It contains clasts of unique rapakivi-textured porphyry from the Chocolate Mountains. The youngest formation showing 250 km of offset along the present San Andreas fault is the lower part of the Pliocene Ridge Basin Group, the offset equivalent of the Punchbowl Formation of Devil's Punchbowl. It contains clasts of "polka dot" granite from north of the Mecca Hills.

Offsets in southern California are in good agreement with measurements in central California and in fair agreement with offsets predicted by plate-tectonic theory for the opening of the Gulf of California.

Faulting and warping have influenced sedimentation along the San Andreas fault and have created local basins such as the Pliocene Ridge basin. But the most important effect of faulting is the gross offset of originally contiguous formations and source terranes and the concomitant juxtaposition of unrelated rocks. Most published studies involving regional basin analysis and paleogeographic reconstructions assume the offset along the San Andreas fault is smaller or occurred earlier and at slower rates than now is demonstrated. Reconstructions based on traditional concepts of the landmasses of Mohavia and Salinia are especially in error. Previous interpretations need to be reevaluated in light of our current knowledge of fault offsets.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA